Consequences of daptomycin-mediated membrane damage in Staphylococcus aureus

Objectives: The proposed lethal action of daptomycin on Staphylococcus aureus results from the loss of K+ and membrane depolarization. However, whether these events alone cause cell death has been questioned. We sought to determine whether other consequences of daptomycin-mediated membrane damage may contribute to cell death. Methods: Previously established assays were used to evaluate the membrane damaging activity of daptomycin at a single time-point of 10 min. More detailed time-course experiments were also performed to determine the kinetics of membrane depolarization and leakage of K+, Mg2+ and ATP. The kinetics of inhibition of macromolecular synthesis following exposure to daptomycin were also determined by assaying the incorporation of radioactive precursors into macromolecules. Results: Daptomycin exhibited no membrane damaging activity in single time-point assays following exposure to the antibiotic for 10 min. Kinetic analysis confirmed these results as leakage of intracellular components did not occur until 20-30 min, membrane depolarization was gradual and cells remained biosynthetically active for at least 30 min after exposure to daptomycin. Viability declined rapidly after exposure to daptomycin and appeared to precede other detectable changes. Conclusions: These data show that daptomycin-induced loss of Mg2+ and ATP occurs in conjunction with the previously reported leakage of K+ and membrane depolarization. We propose that the lethal activity of daptomycin is not simply due to loss of K+ and probably involves more general damage to the membrane.